GD&T for Dummies 3

[maheP]

If i draw a hole on a plate and dimension it from say the bottom and the left hand side, and give it a position tolerance. How does the machinist locate the hole properly to drill it?

I know that there will be a position circle at the intersection of my horizontal and vertical dimensions and the hole center must lie inside this circle, but how can a machinist find this?

Awfully silly question, but all the threads go through the theory and i am familiar with Y14.5-2009, my mind is just not working.

 

[PRuggiero]

maheP, I'll give it a shot.

The machinist will probably machine the perimeter of the plate to ensure all the edges are orthogonal/parallel to each other. Next, he can take an edge finder and hit both edges that you use as your 0,0. This will the center of his tool to that 0,0 point, now he can just go to the coordinates you provided to him with a drill bit and go at it.

Here is the type of edge finder i'm talking about:

http://en.wikipedia.org/wiki/Wiggler_(tool)

Basically it looks very unstable when the machine starts up, but as it comes in contact with the edge it becomes very stable, alerting the machinist that he has hit the edge. Then the diameter of the edge finder is known so the machinist can plug the radius into the machine to align it to the edge. Repeat for the other edge and the center of the spindle is now aligned to the edge.

Hopefully that helps...

 

[maheP]

Thanks for that, I have seen it done years ago with that little wiggly probe mounted on the machine. But if i give say a pitch circle diameter and an angle from 12 o'clock for a hole to be bored, then he will still have to drill a small hole to clock everything up won't he?

And my datums for the position tolerance control frame then (for a circular part) should be from the bottom face and the 12/6 o'clock axis where the basic angular dimensions are taken from on the drawing. Correct me if i am wrong.

I have attached what i am talking about so have a look. I think it needs to be changed from the centre bore to the 12/6 o clock axis on the plate.

 

[powerhound]

maheP,
Regarding your first post: I think you are really overthinking the issue. If you give coordinates from an edge, there are several different ways the machinist may approach it. Using the edge finder is one way. I would never use a wiggler in lieu of an edge finder. The goal of the machinist is not to find the cylindrical tolerance zone. It is to drill the hole where you say to drill it and take every step to make sure it winds up in the tolerance zone. The machinist will move his piece, relative to your datum structure, to the basic dimensions you have provided. The edge finder will get you really close to your X and Y zero. Spot drilling the hole before drilling it will ensure the drill bit doesn't walk as much.

Regarding your second post: Again, I think you are overcomplicating the issue. There is no need to drill extra holes for orientation. If your print is supposed to comply to ASME Y14.5M-2009 then all you have to do is add an MMC modifier to the feature control frame locating the M3 holes and the two patterns must be relative to each other as shown. This is per the Simultaneous Requirements rule. An MMC modifer on a tapped holes seems illogical but it is legal, and I only do it to invoke simultaneous requirements. You should also change how the dimensions are shown and how the datum B is identified.

None of this addresses the orientation of the teeth around the outer edge of the part. A third datum, such as the pattern of 3.4mm holes, will fix this.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[maheP]

powerhound,
Thanks for the feedback. I understand what you mean about the machinist. I was thinking about it backwards, I felt it was not possible ask a machinist to locate the centre of the true position diameter because the dimensions are all basic but he will just drill it and hope for the best in a way.

And the drawings are in the process of going from the standard you see to proper y14.5-2009 standard, what do you mean about the datum B and how it is identified. Also, what do you mean about the teeth on the outer edge? Why do i need a third datum and what FCR should i put it in, im asking because the rest of the drawing is cut off where i have drawn the tooth geometry etc.

Thanks

 

[powerhound]

What I mean by how datum B is identified is shown in the standard. The standard shows several ways to identify a datum feature and this is not one of them. This looks like an ISO drawing with the way the text is shown in the direction of the dimension.

If the teeth are not oriented to the hole pattern functionally then you don't have to do anything. If they are, you need to orient them to something. If you have oriented the teeth to the hole pattern elsewhere on the drawing, then disregard my comments about it.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[maheP]

powerhound,

I understand what you mean, i just moved the datum B to the correct place coincident with the diameter dimension line as shown in fig. 7.4 of y14.5 for example.

You are correct the teeth have been oriented similarly to the holes only its on another view.

Cheers

 

[ModulusCT]

Actually, if you're using linear dimensions to locate your hole and applying your positional tolerance to them rather than using a true position FCF than the tolerance zone is a square, not a circle. Just sayin'...

 

[powerhound]

Modulus,

I don't see where he has used linear dimensions to locate anything. What am I missing?

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[maheP]

ModulusCT,

I have not used any linear dimensions.

 

[Belanger]

FYI, Modulus, even position FCFs in GD&T can create a square tolerance. Simply drop the diameter symbol, and use two FCFs (one for each direction).

Linear dimensions with ± tolerance shouldn't really be used for locating a hole, mainly because there will be ambiguity about datum precedence among the edges of the part.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[ModulusCT]

"FYI, Modulus, even position FCFs in GD&T can create a square tolerance. Simply drop the diameter symbol, and use two FCFs (one for each direction).

Linear dimensions with ± tolerance shouldn't really be used for locating a hole, mainly because there will be ambiguity about datum precedence among the edges of the part."

@Belanger: I always thought that the main issue was due to the square tolerance zone created. If you use true position, you gain about 10-15% more tolerance zone with the same tolerance value. With the square TZ you end up having to toss good parts because the center of the hole falls outside the tolerance zone, but the same error is acceptable (and rightly so) with a circular tolerance zone.

@Powerhound & maheP: I must have assumed something incorrectly. When you said that you dimensioned from the bottom and left hand sides and gave it a positional tolerance, I assumed that meant you applied your tolerances to the linear dimensions. My mistake.

 

[Belanger]

Modulus -- that square vs. circle idea is certainly an advantange of GD&T's position tolerance, but that is only true if the feature control frame includes the diameter symbol. The circle-over-square advantage is not automatic just by speaking the GD&T lanuage.
There are times when we might use GD&T but not show the diameter symbol; by default the tolerance zone's shape would be 2 parallel planes. (That's why you would need two FCFs to really get the square effect.)
If you have a copy of the ASME Y14.5 standard, see Fig. 5-41 of the 1994 edition or Fig. 7-28 of the 2009 edition.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[ModulusCT]

Thanks Belanger!

I'm not a vegetarian because I dislike meat... I'm a vegetarian because I HATE plants!!

 

[SeasonLee]

ModulusCT

I am not quite sure where did you get the figure of 10-15% more tolerance zone.....;actually, its 57% more tolerance zone increased as compared with square tolerance zone.

SeasonLee